This invention relates to a process for the preparation of aminomethylphosphonates, and more particularly, to the preparation of an N-acylaminomethylphosphonic acid.
N-Substituted-aminomethylphosphonic acids are useful intermediates in the preparation of various products, including sequestering agents and herbicides. Thus, for an example, an N-alkyl-N-phosphonomethylglycine, such as N-isopropyl-N-phosphonomethylglycine, can be dealkylated under alkaline conditions to the corresponding N-phosphonomethylglycine using the method disclosed in EPO Patent Application 86 870 047.7.
N-Phosphonomethylglycine, known also by its common name glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling a large variety of weeds. It is applied to the foliage of a very broad spectrum of annual and perennial grasses and broadleaf plants. Industrial uses include control of weeds along roadsides, waterways, transmission lines in storage areas and in other non-agricultural areas. Usually, N-phosphonomethylglycine is formulated into herbicidal compositions in the form of its various salts in solution, preferably water. The process of the present invention can be used to prepare N-acylaminomethylphosphonic acid, which is useful in the synthesis of N-phosphonomethylglycine.
Numerous methods are known to those skilled in the art for the phosphonomethylation of amides. For example, the disclosure of Miller et al. in U.S. Pat. No. 4,657,705 describes a process for the preparation of an N-substituted aminomethylphosphonic acid, comprising reacting substituted amides with phosphorous acid and formaldehyde in an aqueous acidic medium. Under these conditions the amide is readily hydrolyzed to the free amine prior to phosphonomethylation. N-acylaminomethylphosphonic acids are not isolated or formed in this process.
Pulwer and Balthazor in Synthetic Communications, 16(7), 733-739 (1986) report a novel procedure for the preparation of the dimethyl ester of N-benzoylaminomethylphosphonic acid by treating N-hydroxymethylbenzamide with a mixture of phosphorous trichloride and trimethyl phosphite. Hydrolysis of the ester intermediate with acid gave aminomethylphosphonic acid.
U.S. Pat. No. 2,304,156 describes a process for the preparation N-acyl-N-aminomethylphosphonic acid by treatment of a methylol compound with a phosphorus trihalide, and then converting the intermediate ester compound to the phosphonic acid by treatment with water after letting it stand in an enclosed vessel for a long period of time.
Vail, et al., in Journal of Organic Chemistry, 27, pp 2067-2070 (June 1962) report that in general, many N-methylol derivatives of the amide type are unstable products which release formaldehyde on heating, whereas the ethers of these derivatives are more stable.
Polish patent application 117780 discloses a method for preparing aminomethylphosphonic acid by reacting phosphorous trichloride with a methylolamide solution in acetic acid and hydrolyzing the reaction mixture. It reports that the reaction is carried out by adding the methylolamide solution to the phosphorous trichloride. It reports that changing the reaction sequence causes an almost complete reaction of methylolamide to give low yields of aminomethylphosphonic acid after hydrolysis.
Oleksyszyn, et al. reported in Synthesis (June 1978), pages 479 and 480, a method for the preparation of aminoalkanephosphonic acids in which the aminoalkanephosphonic acid is directly produced from phosphorus trichloride or di-chlorophosphines, carbonyl compounds (aldehydes or ketones) and alkyl carbamates. The reference discloses that the replacement of the carbonyl compounds with the corresponding acetals, as well as replacement of carbamates with simpler amides (e.g. acetamide or benzamide,) resulted in decreased yields.
Despite the prediction of low yields in the prior art and considering the problems associated with handling the unstable methylol derivative, Applicants have now found that N-acylaminomethylphosphonic acids can be produced in high yields and purity from acetamides and benzamides without pre-forming the methylol derivative by the process of the present invention.